test: Make more and smaller allocations in shmalloc

The shmalloc test works like this:
 * Randomly select a slot out of 12.
 * If that slot is occupied, free it. If that slot is free, attempt to
   make an allocation of size rand() % total_heap_size.
 * At the end of the test, assert (among other things) that we were able
   to make at least 5 allocations at once given 500000 iterations.

It turns out that getting 5 outstanding allocations at once using this
algorithm is not trivial. I did a simulation with a simpler version of
this that just attempts to make (rand % total) size allocations without
overflowing the max [1], and found that the average number of allocations
you're able to make is about 1.8, with a standard deviation of 0.9. That
means that the test is asserting that in 500000 iterations it will be
able to do >3 standard deviations better than the mean in ~500000/1.8
iterations.

Since we never vary the rand seed, and we happened to get lucky with the
default seed, it all works. But when I try to compile in 32-bit mode, I
think I get an unlucky default seed, and always fail this 5 allocation
test.

There are a few knobs you could twist to fix this:
 * You could reduce the allocation bar below 5.
 * You could increase the iteration count above 500000.
 * You could increase the slot count above 12 so that the first dice
   roll favors trying allocations more.
 * You could decrease the max allocation size from "total" to reduce the
   amount each allocation can occupy.

This change opts for the last option. Reducing the max allocation to
total / 2 increases the (simplified) simulation result to a mean of 3.5
with a standard deviation of 1.27. This means the test now only has to
do better than < 1 standard deviation from the mean in 500000
iterations. This is still probabilistic, but is much less likely to
break if you sneeze on it wrong.

[1] https://paste.googleplex.com/6653229610827776

BUG=b:179062230
BRANCH=none
TEST=make -j runhosttests BOARD=host and x86_64 and i686.

Signed-off-by: Evan Green <evgreen@chromium.org>
Change-Id: Ifb16533ee6fa02ce2613c32b1462b78ef0c6cd21
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3059230
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/test/shmalloc.c b/test/shmalloc.c
index 70e0fb5aab..a596d173e7 100644
--- a/test/shmalloc.c
+++ b/test/shmalloc.c
@@ -253,7 +253,7 @@ void run_test(int argc, char **argv)
 				return;
 			}
 		} else {
-			size_t alloc_size = r_data % (shmem_size);
+			size_t alloc_size = r_data % (shmem_size / 2);
 
 			/*
 			 * If the allocation entry is empty - allocate a